Production of cube texture in sheets and strips of silicon and/or aluminum containing iron alloys



United States Patent 3,207,639 PRODUCTION OF CUBE TEXTURE IN SHEETS AND STRIPS 0F SILICON AND/OR ALUML NUM CONTAINING IRON ALLOYS Hans-Eberhard Miibius, Altena, Westphalia, Germany (Nordring 72, 662 Volklingen, Saar, Germany) No Drawing. Filed Feb. 14, 1961, Ser. No. 89,105 Claims priority, application Germany, Feb. 16, 1960, V 18,060 4 Claims. (Cl. 148-113) The present invention relates to an improved process for the production of sheets, strips or ribbons having a well developed oriented texture known as cube texture which has a (100) [001] orientation from silicon and/ or aluminum containing iron alloys.

The steadily increasing use of electrical energy renders it necessary to find magnetic iron alloys for equipping electric motors and other apparatus which at high magnetic flux densities exhibit as small losses as possible. These requirements are met by alloys of iron with nickel or silicon and/ or aluminum which are oriented cubically. Each cube has three favorable magnetizing directions which are perpendicular to each other. If the magnetic field runs parallel to one of these directions only a minimum of energy is required to magnetize the workpiece in question.

In simply oriented sheets or strips of silicon containing iron one of the cube edges, which is the direction of easy magnetization, lies in the rolling direction whereas magnetization can only be effected with difliculty in a direction transverse to the rolling direction. This orientation is known as the Goss-texture or the (110) [001] texture. Such texture is obtained in steels with certain silicon content by hot working followed by several cold working stages with intermediate anneals, as well as by a final recrystallization anneal at 7001300 C., preferably in a hydrogen atmosphere (see US. Patent 1,965,- 559). Sheets of such materials are employed in the construction of electric motors and apparatus and, for example, have a U-shape. With such material one, however, must be content that either only the legs or the crosspiece of the stamped U-shaped sheets are easily magnetized.

This disadvantage is avoided in sheets or strips which have a (100) [001] orientation or the cube texture. Such orientation is characterized in that magnetization can be effected easily both in the rolling direction and the direction transverse to the rolling direction. Such orientation has been described in US. Patent 2,875,114. Both of the orientations referred to above are obtained in sheets and strips of alloys of iron containing 0.5-3.5% of silicon or 05-25% of aluminum or 0.53.5% of both such alloying elements together The present invention concerns a process for the production of such sheets and strips from the iron alloys indicated and is particularly concerned with rules under which the final recrystallization anneal is to be carried out.

Various measures for the production of the described highly magnetizable semi-finished products, such as sheets, strips or ribbons or stampings therefrom, have been known in part and have been proposed in part. They concern specifications for the melting down, the rolling while maintaining certain degrees of reduction, favorable number of passes, intermediate anneal temperatures, annealing periods, ageing periods as well as the temperature of the final recrystallization anneal and the atmospheres which are advantageously employed.

Even when all of the known measures favoring attainment of a pronounced or well developed cube texture are employed it was found that the proportion of the grains 3,207,030 Patented Sept. 21, 1965 ice oriented in the cube texture could be increased substantially and that such orientation can be attained even in thicker bands or sheets if, according to the invention, the final recrystallization anneal is carried out at the usual temperature under certain prescribed conditions, namely, changing the atmosphere before the end annealing temperature is reached or heating the material to end annealing temperature extremely rapidly to effect a shock-like heating and simultaneously providing a silicon or aluminum vapor pressure in the annealing atmosphere which is greater than that of the silicon and/ or aluminum in the material being annealed in order to prevent thermal erosion at the grain boundaries during the anneal.

As is well known, the structure of a wrought metal is capable of recrystallization. Such recrystallization effects a new grain formation with or without a change in texture. When'a secondary recrystallization follows the primary recrystallization, as is the case in the formation of the Goss texture and the pronounced cube texture, such new texture formation is especially apparent when the normal grain growth is hindered and only certain grains are permitted to grow. As a rule they possess a certain orientation. The normal grain growth, for example, can be hindered by the presence of impurities at the grain boundaries. Use is made of this in the secondary recrystallization for the production of the Goss texture. On the other hand, care must be taken that the presence of too great a quantity of impurities does not hinder or prevent a secondary recrystallization.

Thus far, the way to the production of a cube texture by secondary recrystallization was via the production of a well defined preferred grain orientation, namely, the production of the Goss texture (110) [001].

The present invention concerns a new way of producing the cube texture, namely, by carrying out the final anneal under certain controlled conditions.

According to the invention the atmosphere is changed during heating of the material to the temperature employed for the final anneal rather than. using the same atmosphere as previously has been customary. The gases or gas mixture employed during the initial heating period, that is, before the change in atmosphere, are so selected that they have a carburizing, nitriding or oxidizing action thereon. Argon, a vacuum of 0.1 to 0.005 mm. Hg, an atmosphere of H CH and in some instances nitrogen have proved suitable as atmospheres before the change in atmosphere. It is advantageous if the change in atmosphere is effected before the actual annealing temperatures which, as is known, lie between 1000 and 1400 C. are reached. With slow heating the change in atmosphere is effected at a temperature between 600 to 1200 C., preferably, between 700 and 1000 C. The selection of the temperature at which the change in atmosphere is effected depends upon the type of gas atmosphere employed and the material being treated and therefore also upon the quantity of impurities present. It is also important in carrying out such final anneal according to the invention to heat the material to be annealed rapidly before the atmosphere is changed, for example, to about 800 C. in 2 hours and to heat such material to the actual annealing temperature after the change in atmosphere more slowly. After the change in atmosphere at high vacuum or hydrogen should be used as the atmosphere. The rate of heating before the change in atmosphere (fast heating) should be about 500 to 1000" C. per hour, for example, 900 C. per hour. The slower heating after the change in atmosphere should be at a rate of about 10 to C. per hour, for example, 30 C. per hour.

It is also, according to the invention, possible to employ a shock-like heating for the final annealing temperature instead of the previously described manner of heating with change in atmosphere so that the end an- =9 nealing temperature is reached in a substantially shorter period of time than in the above-described procedure. Such shock-like heating is carried out at a rate between 2000 and 20,000 C. per hour, for example, 3800 C. per hour. When such extremely rapid heating is employed it is not absolutely necessary to effect a change in atmosphere. For example, the material to be annealed can be heated inductively in a hot oven to 1250 C. in 25 minutes under high vacuum and annealed at such temperature for 12 hours.

The final recrystallization anneal according to the invention, both the slow and fast procedures, can also be carried out in suitably designed pusher type furnaces and rotary furnaces containing one or more chambers and capable of providing a protective atmosphere for the material treated.

For example, a furnace for the slower procedure can be operated with argon under reduced pressure or another gas atmosphere of the described type in its first chamber, a medium vacuum of 10 to 1 mm. Hg in its second chamber, a fine vacuum of 1 to 10- mm. Hg in its third chamber and a high vacuum of 10" to l mm. Hg in its fourth chamber. The first cooling can also be effected in the last chamber. When a protective gas such as helium or hydrogen is employed in the cooling zone, it is preferable to keep the gas in motion to obtain a better cooling action or to pump it out of the furnace and after cooling, for example, by heat exchange with the heating zone, recycling it to the cooling zone. In view of the long annealing period at high temperature required for the secondary recrystallization, the annealing of the band or sheet preferably is carried out in the bundle or coiled form.

There also is the possibility of etfecting the change in atmosphere after the end temperature of the final recrystallization anneal is reached.

A further measure which is employed according to the invention during the final recrystallization anneal is to provide a sufficiently high aluminum and/or silicon partial pressure in the surrounding atmosphere that escape of silicon and/ or aluminum present at the grain boundaries cannot occur so that pitting along the grain boundaries is prevented. This can, for example, be achieved by surrounding the material being annealed with an iron silicon and/or aluminum alloy having a higher silicon and/or aluminum content so as to limit vaporization of such elements from the material being treated.

It is also possible to prevent vaporization of the aluminum and/or silicon from the grain boundaries of the material being annealed by introducing silicon and/ or aluminum into the atmosphere employed for the final anneal by introducing pure silicon and/or aluminum, which, if necessary, has been subjected to a pretreatment under high vacuum at temperatures higher than those employed for the anneal to improve the purity thereof, directly in the furnace space alongside the sheet or band being annealed. It is also possible to mix finely divided aluminum and/ or silicon with the thermally stable oxides usually employed for heat insulation such as A1 0 and MgO and place such mixture between the layers of the material to be annealed. The vapor pressure of the aluminum and/ or silicon can be varied according to necessity by locally heating the aluminum and/ or silicon to a higher or lower temperature than that of the material being annealed or by employing alloys of silicon and/ or aluminum with a greater or lesser quantity of iron or another element. It is also possible to supply the silicon by introducing a silicon compound such as a silane, for example, Sit-I which decomposes at higher temperatures, into the annealing atmosphere.

The roughness of the normal cold rolled sheets and ribbons, as in the case of intergranular corrosion caused by vaporization of silicon and/or aluminum from the grain boundaries, also hinders the secondary recrystallization during the final anneal. For this reason it is preferable, according to the invention, to smooth such cold rolled sheets or ribbons before subjecting them to the final recrystallization anneal, for example, by subjecting the cold rolled sheets or ribbons to a finishing operation, such as a skin pass between a pair of relatively large rollers with especially smooth surfaces. In addition, it may be desirable to polish the material to be annealed chemically or electrolytically. It was found that an electrolyte composed of 1 part by weight of chromic acid anhydride (CrO and 9 parts by weight of orthophosphoric acid was well suited as an electrolyte for the electrolytic polishing of silicon steels. The temperature of such electrolyte during the electrolytic polishing preferably is about C., the sheet or ribbon to be polished being made the anode and being subjected to a current density of 0.3 amp/emi A copper sheet can be used as the cathode. The time required for the electrolytic polishing lies between 1 and about 10 minutes, preferably about 5 minutes.

The following examples will serve to illustrate the process according to the invention.

In such examples, the material given the final recrystallization anneals according to the invention was an alloy of iron with 2.5% of silicon (containing the usual impurities of the iron) which had been hot rolled to a rib bon, annealed and then cold rolled in two stages with an intermediate anneal to a thickness of 0.35 mm., the degree of deformation in the first cold rolling stage being 85% and in the second stage 65%.

Example 1 The cold rolled ribbon was heated in about 1 hour to 900 C. in argon containing 0.015 vol. percent of oxygen and then heated to 1250 C. under a high vacuum (10* mm. Hg) in 10 hours. This temperature was maintained for 12 hours and the material was then cooled down under high vacuum. 80 vol. percent of the thus annealed ribbon was oriented in the cube texture.

Example 2 The cold rolled ribbon was electropolished using the electrolyte and procedure described above and then immediately heated under vacuum to 1250 C. in 20 minutes. After such temperature was reached the pressure was adjusted to below 10- mm. Hg within 10 minutes. Such vacuum was maintained over the entire annealing period of 10 hours. The annealed ribbon was cooled under vacuum and 90 vol. percent of the thus annealed ribbon was oriented in the cube texture.

Example 3 The cold rolled ribbon was polished chemically and given a skin pass between a pair of very smooth relatively large rollers and heated in about 1 hour to 700 C. under a vacuum of 5X10 mm. Hg and then in about 18 hours to 1250 C. under a high vacuum of 10 mm. Hg. Ferrosilicon of about the composition FeSi was placed alongside the ribbon being annealed and heated along therewith. The annealing period was 24 hours. The cooling was effected under hydrogen. vol. percent of the thus annealed ribbon was oriented in the cube texture.

I claim:

1. A process for producing [001] oriented cube texture in sheets of a magnetizable iron alloy selected from the group consisting of iron-silicon alloys containing 0.5 to 3.5% of silicon, iron-aluminum alloys containing 0.5 to 2.5% of aluminum and iron-silicon-l-aluminum alloys in which the silicon+aluminum content is 0.5 to 3.5% comprising hot rolling sheets of said alloy, cold rolling said hot rolled sheets, heating up the cold rolled sheets essentially consisting of said iron alloy in a final recrystallization anneal at a rate of about 500 to 1000 C. per hour to a temperature in the range of about 700 C. to 1000 C. in an atmosphere essentially consisting of argon and about 0.015 vol. percent of oxygen, then changing the atmosphere to a high vacuum of a pressure between and l0 mm. Hg, then heating up said sheets further at a rate of about 10 C. to 100 C. per hour to an annealing temperature between 1000 and 1400 C. under said high vacuum, the duration of the said heating up of the cold rolled sheets to the annealing temperature being a period of over an hour, and maintaining said sheets at said annealing temperature under said high vacuum until completion of the final recrystallization anneal.

2. The process of claim 1 in which a vapor pressure of the alloying component of said iron alloy selected from the group consisting of silicon and aluminum in the atmosphere surrounding the iron alloy during the final secondary recrystallization anneal is maintained greater than that of said alloying component in said alloy.

3. A process for producing (100) [001] oriented cube texture in sheets of a magnetizable iron alloy selected from the group consisting of iron-silicon alloys containing 0.5 to 3.5% of silicon, iron-aluminum alloys containing 0.5 to 2.5% of aluminum and iron-silicon+aluminum alloys in which the silicon-i-aluminum content is 0.5 to 3.5% comprising hot rolling sheets of said alloy, cold rolling said hot rolled sheets, heating up the cold rolled sheets essentially consisting of said iron alloy in a final recrystallization anneal at a rate of about 500 to 1000 C. per hour to a temperature in the range of about 700 C. to 1000" C, under a vacuum of about 0.1 to 0.005 mm. Hg, then increasing the vacuum to a high 6 vacuum of a pressure between 10- and 10- mm. Hg, then heating up said sheets further at a rate of about 10 C. to C. per hour to an annealing temperature between 1000 and 1400 C. under said high vacuum, the duration of the said heating up of the cold rolled sheets to the annealing temperature being a period of over an hour, and maintaining said sheets at said annealing temperature under said high vacuum until completion of the final recrystallization anneal.

4. The process of claim 3 in which a vapor pressure of the alloying component of said iron alloy selected from the group consisting of silicon and aluminum in the atmosphere surrounding the iron alloy during the final secondary recrystallization anneal is maintained greater than that of said alloying component in said alloy.

References Cited by the Examiner UNITED STATES PATENTS 2,410,220 10/46 Langworthy 148-6 X 2,875,113 2/59 Fitz 148113 2,965,526 12/60 Wiener 148113 X 2,992,952 7/ 61 Assmus et a1. 148-411 3,090,711 5/63 Kohler 148-111 FOREIGN PATENTS 825,903 12/59 Great Britain.

DAVID L. RECK, Primary Examiner.

MARCUS U. LYONS, RAY K. WINDHAM, Examiners. 

1. A PROCESS FOR PRODUCING (100) (001) ORIENTED CUBE TEXTURE IN SHEETS OF A MAGNETIZABLE IRON ALLOY SELECTED FROM THE GROUP CONSISTING OF IRON-SILICON ALLOYS CONTAINING 0.5 TO 3.5% OF SILCION, IRON-ALUMINUM ALLOYS CONTAINING 0.5 TO 2.5% OF ALUMINUM AND IRON-SILICON+ALUMINUM ALLOYS IN WHICH THE SILICON+ALUMINUM CONTENT IS 0.5 TO 3.5% COMPRISING HOT ROLLING SHEETS OF SAID ALLOY, COLD ROLLING SAID HOT ROLLED SHEETS, HEATING UP THE COLD ROLLED SHEETS ESSENTIALLY CONSISTING OF SAID IRON ALLOY IN A FINAL RECRYSTALLIZATION ANNEAL AT A RATE OF ABOUT 500 TO 1000*C. PER HOUR TO A TEMPERTURE IN THE RANGE OF ABOUT 700*C. TO 1000*C. IN AN ATMOSPHERE ESSENTIALLY CONSISTING OF ARGON AND ABOUT 0.015 VOL. PERCENT OF OXYGEN, THEN CHANGING THE ATMOSPHERE TO A HIGH VACUUM OF A PRESSURE BETWEEN 10**3 AND 10**6 MM. HG, THEN HEATING UP SAID SHEETS FURTHER AT A RATE OF ABOUT 10*C. TO 100*C. PER HOUR TO AN ANNEALING TEMPERATURE BETWEEN 1000 AND 1400*C. UNDER SAID HIGH VACUUM, THE DURATION OF THE SAID HEATING UP OF THE COLD ROLLED SHEETS TO THE ANNEALING TEMPERATURE BEING A PERIOD OF OVER AN HOUR, AND MAINTAINING SAID SHEETS OF SAID ANNEALING TEMPERATURE UNDER SAID HIGH VACUUM UNTIL COMPLETION OF THE FINAL RECRYSTALLIZATION ANNEAL. 